Production of propylene glycol monoalkyl ether

ABSTRACT

The invention is a process for producing a propylene glycol monoalkyl ether which comprises first reacting propylene oxide and an alcohol in the presence of an alkali or alkaline earth metal alkoxide catalyst to produce an alkoxylation mixture comprising propylene glycol monoalkyl ether; distilling the alkoxylation mixture to produce a first overhead stream comprising propylene oxide and alcohol and a first bottoms stream comprising propylene glycol monoalkyl ether; and then distilling the first bottoms stream to produce purified propylene glycol monoalkyl ether as a second overhead stream. The reaction and/or one or more of the distillation steps occur in the presence of an alkali metal borohydride. The process produces propylene glycol monoalkyl ether with low UV absorbance and reduced carbonyl impurities.

FIELD OF THE INVENTION

This invention relates to a process for producing a propylene glycolmonoalkyl ether.

BACKGROUND OF THE INVENTION

Propylene glycol monoalkyl ethers are high-performance industrialsolvents for paints and coatings, cleaners, inks, and a variety of otherapplications, including agricultural, cosmetic, electronic, textile, andadhesive products. They are also used as chemical intermediates forend-products such as propylene glycol ether acetates.

Typically, propylene glycol monoalkyl ethers are formed by the reactionof propylene oxide with an alcohol, such as methanol or 1-butanol.Although a catalyst is not required, the reaction is typically performedin the presence of a catalyst. A wide variety of catalysts and reactionconditions are taught in the prior art.

The catalysts used in this process include acidic, basic, and neutralspecies. Particularly useful catalysts include acids such as sulfuricacid, boric acid and some fluorine-containing acids; or bases such asalkali and alkaline earth metal hydroxides and alkoxides, tertiaryamines, and certain metal oxides. G.B. Pat. No. 271,169, for instance,discloses the use of sulfuric acid, alkali metal alkoxides, and alkalimetal salts of lower fatty acids. U.S. Pat. No. 2,327,053 teaches theuse of metal halides such as stannic halides, antimony pentahalides,aluminum halides, zinc halides and ferric halides.

A problem associated with these reactions, and in particular the use ofalkali or alkaline earth metal alkoxide catalysts, is that the propyleneglycol monoalkyl ether product is contaminated with various carbonylimpurities (such as formaldehyde, acetaldehyde, propionaldehyde,acetone, methoxy acetone, and methoxy butenone) that lead to high UVabsorption. For particular applications, it may be necessary to limitthe amount of carbonyl impurities and thus lower the UV absorbance ofthe propylene glycol monoalkyl ether product.

In sum, new processes to produce propylene glycol monoalkyl ethers areneeded. Particularly useful processes will decrease the amount ofcarbonyl impurities and thus improve the UV absorbance and color of thepropylene glycol monoalkyl ether product. We have discovered aneffective, convenient process that produces propylene glycol monoalkylether having low UV absorbance.

SUMMARY OF THE INVENTION

The invention is a process which comprises: (a) reacting propylene oxideand an alcohol in the presence of an alkali or alkaline earth metalalkoxide catalyst to produce an alkoxylation mixture comprisingpropylene glycol monoalkyl ether; (b) distilling the alkoxylationmixture to produce a first overhead stream comprising propylene oxideand the alcohol and a first bottoms stream comprising propylene glycolmonoalkyl ether; and (c) distilling the first bottoms stream to producepurified propylene glycol monoalkyl ether as a second overhead stream,wherein the reaction and/or one or more of the distillation steps occurin the presence of alkali metal borohydride. The process surprisinglyreduces the carbonyl impurity level and UV absorbance in the propyleneglycol monoalkyl ether.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention comprises first reacting a reaction mixturecomprising propylene oxide and an alcohol in the presence of an alkalior alkaline earth metal alkoxide to produce an alkoxylation mixturecomprising a propylene glycol monoalkyl ether. In this reaction, twopropylene glycol monoalkyl ether isomers, 1-alkoxy-2-propanol and2-alkoxy-1-propanol, may be produced. For instance the reaction ofpropylene oxide and methanol produces both 1-methoxy-2-propanol (knownas “PM-1”) and 2-methoxy-1-propanol (“PM-2”). PM-1, the major productfrom methanol propoxylation, is the isomer sold commercially.

The process to produce propylene glycol monoalkyl ethers is well knownand propylene glycol monoalkyl ethers are commercially availableproducts. Commercial products include Lyondell Chemical Company'sARCOSOLV® propylene glycol ethers, such as ARCOSOLV PM (propylene glycolmonomethyl ether), ARCOSOLV PNB (propylene glycol normal butyl ether),ARCOSOLV PTB (propylene glycol tertiary butyl ether), and ARCOSOLV PNP(propylene glycol normal propyl ether).

The reaction mixture comprises propylene oxide and an alcohol. Thealcohol used for the reaction is suitably an aliphatic, cycloaliphaticor an aromatic alcohol and may have one, two, or more hydroxyl groups.Preferably, the alcohol has only one hydroxyl group. The alcohol may beprimary, secondary or tertiary in structure, and may be saturated orunsaturated as well as substituted with various substituents. Mostpreferably, the alcohol is a C₁-C₄ alcohol, particularly, methanol,ethanol, 1-propanol, 2-propanol, 1-butanol, and tert-butanol.

The relative amounts of propylene oxide and alcohol that make up thereaction mixture can vary over a fairly wide range. Usually, however, itis preferred to use at least one mole of alcohol for every propyleneoxide equivalent. For example, when methanol reacts with propylene oxideto produce propylene glycol monomethyl ether, it is preferred to useabout 2 moles of methanol per mole of propylene oxide. Preferably, themolar ratio of alcohol to propylene oxide is at least 1.1:1 and is morepreferably in the range from 1.5:1 to 5:1.

Although not necessary for the reaction, the reaction mixture may alsoinclude a solvent. Suitable solvents include C₅-C₂₀ aliphatichydrocarbons such as hexane, C₆-C₂₀ aromatic hydrocarbons such astoluene, nitriles such as acetonitrile, and ethers such as methylt-butyl ether.

The reaction of propylene oxide with an alcohol is performed in thepresence of an alkali or alkaline earth metal alkoxide catalyst.Although any alkali or alkaline earth metal alkoxide may be used(lithium, sodium, potassium, rubidium, cesium, magnesium, calcium,strontium, or barium alkoxides), alkali metal alkoxides are preferred.Potassium alkoxides and sodium alkoxides are particularly preferred.Preferred sodium alkoxides include sodium methoxide, ethoxide,n-propoxide, and n-butoxide, and t-butoxide. Preferred potassiumalkoxides include potassium methoxide, ethoxide, n-propoxide,n-butoxide, and t-butoxide. Preferably, the alkoxide corresponds to thealcohol used in the reaction, that is, if the alcohol is methanol thenthe alkoxide is a methoxide such as sodium methoxide or potassiummethoxide.

The reaction of propylene oxide and alcohol to form propylene glycolmonoalkyl ether is preferably carried out at a temperature in the rangeof from about 50° C. to about 250° C., more preferably from about 100°C. to about 180° C. The reaction between propylene oxide and the alcoholis exothermic, so it may be desirable to apply cooling to the reactionmixture in order to control the reaction temperature. The reaction ispreferably carried out at atmospheric pressure or at higher pressure upto about 3000 psig (20,786 MPa).

The reaction step of the invention includes batch, semi-batch, andcontinuous processes. In a typical batch reaction, the reactants (exceptfor the catalyst) are charged to a reactor, catalyst is introduced, andthe mixture is heated to the desired reaction temperature to form analkoxylation mixture comprising propylene glycol monoalkyl ether. In atypical continuous reaction, streams of the propylene oxide, alcohol,any recycle streams, and alkali or alkaline earth metal alkoxidecatalyst are fed continuously into a heated reaction zone to form analkoxylation mixture comprising propylene glycol monoalkyl ether whichis continuously withdrawn from the reactor.

Following reaction, the alkoxylation mixture is subjected todistillation in order to produce a propylene glycol monoalkyl etherproduct. The distillation steps include first distilling thealkoxylation mixture to produce a first overhead stream comprisingpropylene oxide and alcohol and a first bottoms stream comprisingpropylene glycol monoalkyl ether; and then distilling the first bottomsstream to produce purified propylene glycol monoalkyl ether as a secondoverhead stream.

The first distillation step, distilling the alkoxylation mixture, may beoperated at any temperature and pressure which will afford a firstdistillation bottoms stream that contains a higher purity of propyleneglycol monoalkyl ether than was contained in the alkoxylation mixturefrom the reaction. Prior to distillation, the alkoxylation mixture maybe sent to a storage unit and/or a finishing drum in order to removelights from the alkoxylation mixture, however such a storage unit orfinishing drum is not necessary for the process of the invention. If thealkoxylation mixture is sent to a finishing drum, the system pressure inthe finishing drum is reduced such that a significant amount ofunreacted propylene oxide is flashed and vented from the alkoxylationmixture prior to distillation. This flashed propylene oxide may berecycled back to the reaction.

The second distillation step, distilling the first bottoms stream, maybe operated at any temperature and pressure which will afford purifiedpropylene glycol monoalkyl ether as the second overhead stream. Thepurified propylene glycol monoalkyl ether may be a mixture of1-alkoxy-2-propanol and 2-alkoxy-1-propanol. The purified propyleneglycol monoalkyl ether may also be purified 1-alkoxy-2-propanol, whereinany 2-alkoxy-1-propanol produced is separated in the second bottomsstream.

In general, the distillation towers (also referred to as columns) may beof conventional design. The towers are preferentially packed withconventional packing. The temperature and pressure in the towers may beadjusted depending on the type of propylene glycol monoalkyl etherproduced.

The first distillation tower is used in order to remove alcohol andpropylene oxide as an overhead stream and concentrate propylene glycolmonoalkyl ether in the bottoms stream. Preferably, the firstdistillation tower is operated at a temperature of from about 50° C. toabout 200° C. and a pressure of from about 1 psig (108 MPa) to about 150psig (1136 MPa).

The overhead from the first distillation tower (containing unreactedalcohol and propylene oxide) is preferentially recycled to the reactionstep for further reaction. The first distillation bottoms, comprisingpropylene glycol monoalkyl ether, exits the first tower and istransferred to the second distillation tower for further purification ofthe propylene glycol monoalkyl ether.

In the second distillation tower, the first distillation bottoms streamis distilled. The overhead stream from the second distillation towercomprises high purity propylene glycol monoalkyl ether. The seconddistillation bottoms stream, comprising heavier byproducts (such asdipropylene glycol ethers and tripropylene glycol ethers), andoptionally 2-alkoxy-1-propanol where 1-alkoxy-2-propanol is purified inthe overhead stream, may be recycled back to the reactor or purifiedfurther to produce other products. In general, the second distillationtower is operated at a temperature of from about 100° to about 200° C.and a pressure of from about 1 mm Hg (0.133 MPa) to about 760 mm Hg(101.3 MPa).

The purity of the propylene glycol monoalkyl ether stream exiting as anoverhead stream from the second distillation column is usually in therange from about 99 to about 99.95 percent.

Without the addition of an alkali metal borohydride to the reactionand/or distillation steps, the propylene glycol monoalkyl ether producttypically contains by weight greater than 50 ppm of various carbonylimpurities and gives a UV absorbance (at 245 nm) of greater than 1.Typically, the propylene glycol monoalkyl ether produced with no alkalimetal borohydride comprises by weight about 50 to 2,000 ppm of variouscarbonyl impurities, usually about 50 to 1,000 ppm.

Thus, the process of the invention requires that the reaction and/or oneor more of the distillation steps occur in the presence of an alkalimetal borohydride. The presence of alkali metal borohydride surprisinglyreduces the carbonyl concentration and UV absorption in the propyleneglycol monoalkyl ether product. Although any alkali metal borohydridemay be used (lithium, sodium, potassium, rubidium, and cesiumborohydride), sodium and lithium borohydride are preferred, and sodiumborohydride is especially preferred.

If an alkali metal borohydride is added to the reaction mixture, thealkali metal borohydride is preferably added to the reaction mixturesuch that the reaction mixture comprises from about 0.05 to 100 ppmalkali metal borohydride, more preferably from about 0.1 to 50 ppmalkali metal borohydride.

If an alkali metal borohydride is added to the first distillation step,the alkali metal borohydride may be added into the distillation columnas a separate stream from the alkoxylation mixture or may be added intothe alkoxylation mixture prior to the first distillation step.Preferably, the alkali metal borohydride is added into the alkoxylationmixture prior to the first distillation step. The alkali metalborohydride is preferably added to the alkoxylation mixture such thatthe alkoxylation mixture comprises from about 0.1 to 1,000 ppm alkalimetal borohydride, more preferably from about 1 to 100 ppm alkali metalborohydride.

If an alkali metal borohydride is added to the second distillation step,the alkali metal borohydride may be added into the distillation columnas a separate stream from the first bottoms stream or may be added intothe first bottoms stream prior to the second distillation step.Preferably, the alkali metal borohydride is added into the first bottomsstream prior to the second distillation step. The alkali metalborohydride is preferably added to the first bottoms stream such thatthe first bottoms stream comprises from about 1 to 10,000 ppm alkalimetal borohydride, more preferably from about 1 to 100 ppm alkali metalborohydride.

Although an alkali metal borohydride may be added to the reaction stepand both distillation steps, it is preferable to add alkali metalborohydride to the first and/or the second distillation step. It isespecially preferred to add the alkali metal borohydride to the firstdistillation step.

Following the process of the invention, a purified propylene glycolmonoalkyl ether product having a decreased carbonyl impurities contentis produced. Preferably, the purified propylene glycol monoalkyl etherproduct has a UV absorbance, at 245 nm, of 1 or less.

The following examples merely illustrate the invention. Those skilled inthe art will recognize many variations that are within the spirit of theinvention and scope of the claims.

EXAMPLE 1 Reaction Runs

Comparison Run 1A: Methanol (132 g), propylene oxide (110 g), and a2-methoxy-1-propanol recycle mixture (60 g) are added to a stainlesssteel reactor and the temperature of the reaction mixture is raised to130° C. A potassium methoxide catalyst mixture (1.27 g, of 25 wt. %KOCH₃ in methanol) is added to the reaction mixture and stirred. Samplesare taken from the reaction mixture periodically (at 1 h and 10 min andat 2 hr and 10 min after catalyst addition) and analyzed for carbonylcontent and UV absorbance. Results are shown in Table 1.

Run 1B: Run 1B is run according to the procedure of Comparison Run 1Awith the exception that NaBH₄ is added to the reaction mixture with themethanol, resulting in a 0.3 ppm NaBH₄ concentration in the reactionmixture. Results are shown in Table 1.

The results show that a small amount of sodium borohydride in thereaction mixture leads to a reduced amount of carbonyl impurities andreduced UV absorbance.

EXAMPLE 2 Batch Distillation Runs

Comparison Run 2A: A 1-methoxy-2-propanol reaction solution (1198.3 g,containing 778.19 g PM-1) is distilled under vacuum using an 8 mm I.D.spinning band batch distillation column (B/R Instrument Corporation,Easton Md.). The condenser pressure is controlled at 368 mm Hg (49 MPa)and the reflux ratio is set at 50 to 1. This results in a boilingtemperature of 105° C. in the distillation reboiler pot. Samples of theoverhead, purified PM-1 are taken in 20 mL increments and analyzed forcarbonyl content and UV absorbance (at 245 nm). Results are shown inTable 2.

Run 2B: Run 2B is run according to the procedure of Comparison Run 2Awith the exception that 200 ppm NaBH₄ (0.24 g) is added to the1-methoxy-2-propanol solution (1200 g, containing 779.29 g PM-1).Results are shown in Table 2.

The results show that the addition of sodium borohydride into the batchdistillation pot leads to a reduced amount of carbonyl impurities andreduced UV absorbance for the first 40 mL of PM-1 collected overhead,until the sodium borohydride in the pot was depleted after about 7 hoursat 105° C.

EXAMPLE 3 Continuous Distillation Runs

Comparison Run 3A: A 1-methoxy-2-propanol reaction solution, containing65 wt. % PM-1, is purified by continuous distillation under vacuum usinga distillation column (1 in. I.D.×4 ft) containing PRO-PAK® packing. The1-methoxy-2-propanol reaction solution is fed into the distillationcolumn at 18 inches from the bottom of the column and the temperature atthe feeding tray is maintained at 118-119° C. during the distillationprocedure. Feed rate is adjusted to reach a desired liquid residencetime. The reboiler liquid volume is controlled at 220 mL and pottemperature is maintained at 121-122° C. The reflux ratio is controlledat 5 to 1. Purified PM-1 product is withdrawn continuously overhead.PM-1 product samples are analyzed for carbonyl content and UV absorbance(at 245 nm). Results are shown in Table 3.

Run 3B: Run 3B is run according to the procedure of Comparison Run 3Awith the exception that 42 ppm NaBH₄ is added to the1-methoxy-2-propanol reaction solution. Results are shown in Table 3.

The results show that the addition of sodium borohydride into thecontinuous distillation reduces both carbonyl impurities and UVabsorbance for the PM-1 product collected overhead.

TABLE 1 Reaction Run Data Carbonyl Amount Run # Sample Time (ppm) UV,abs. 1A* 1:10 28 1.05 1B 1:10 26 0.87 1A* 2:10 36 1.64 1B 2:10 30 1.3*Comparative Example

TABLE 2 Distillation Run Data Carbonyl PM-1 Collected Amount Run #Overhead (mL) (ppm) UV, abs. 2A* 20 1209 3.87 2B 20 10 0.16 2A* 40 6093.21 2B 40 8 0.15 2A* 60 351 2.66 2B 60 388 0.86 *Comparative Example

TABLE 3 Distillation Run Data Carbonyl Residence Amount Run # Time(minute) (ppm) UV, abs. 3A* 115 58 0.96 3B 119 6 0.16 3A* 59 54 0.96 3B68 9 0.16 3A* 42 76 1.01 3B 46 21 0.21 *Comparative Example

We claim:
 1. A process for producing a propylene glycol monoalkyl etherwhich comprises: (a) reacting propylene oxide and an alcohol in thepresence of an alkali or alkaline earth metal alkoxide catalyst toproduce an alkoxylation mixture comprising propylene glycol monoalkylether and a component having a carbonyl functional group; (b) distillingthe alkoxylation mixture at a temperature of from about 50° C. to about200° C. and a pressure of from about 1 psig to about 150 psig to producea first overhead stream comprising propylene oxide and the alcohol and afirst bottoms stream comprising propylene glycol monoalkyl ether; and(c) distilling the first bottoms stream to produce purified propyleneglycol monoalkyl ether as a second overhead stream, wherein the reactionand/or one or more of the distillation steps occur in the presence of0.05 to 1000 ppm, based upon the total weight of the alkoxylationmixture, of an alkali metal borohydride, and wherein the alkali metalborohydride reduces the concentration of the component having a carbonylfunctional group.
 2. The process of claim 1 wherein the alcohol is aC₁-C₄ alcohol.
 3. The process of claim 1 wherein the alcohol is selectedfrom the group consisting of methanol, 1-propanol, 1-butanol, andtert-butanol.
 4. The process of claim 1 wherein the alkali or alkalineearth metal alkoxide catalyst is an alkali metal alkoxide.
 5. Theprocess of claim 4 wherein the alkali metal alkoxide catalyst is apotassium alkoxide or a sodium alkoxide.
 6. The process of claim 5wherein the potassium alkoxide is selected from the group consisting ofpotassium methoxide, potassium n-propoxide, potassium n-butoxide, andpotassium t-butoxide.
 7. The process of claim 5 wherein the sodiumalkoxide is selected from the group consisting of sodium methoxide,sodium n-propoxide, sodium n-butoxide, and sodium t-butoxide.
 8. Theprocess of claim 1 wherein the alkali metal borohydride is sodiumborohydride.
 9. The process of claim 1 wherein the first overhead streamis recycled back to the reaction step (a).
 10. The process of claim 1wherein the propylene glycol monoalkyl ether is propylene glycolmonomethyl ether.
 11. The process of claim 1 wherein the alkali metalborohydride is added into the alkoxylation mixture prior to distillationstep (b).
 12. The process of claim 1 wherein the alkali metalborohydride is added into the first bottoms stream prior to distillationstep (c).
 13. The process of claim 1 wherein the purified propyleneglycol monoalkyl ether has a UV absorbance, at 245 nm, of 1 or less. 14.A process for producing a propylene glycol monomethyl ether whichcomprises: (a) reacting propylene oxide and methanol in the presence ofan alkali metal methoxide catalyst to produce an alkoxylation mixturecomprising propylene glycol monomethyl ether and a component having acarbonyl functional group; (b) distilling the alkoxylation mixture at atemperature of from about 50° C. to about 200° C. and a pressure of fromabout 1 psiq to about 150 psiq to produce a first overhead streamcomprising propylene oxide and methanol and a first bottoms streamcomprising propylene glycol monomethyl ether; and (c) distilling thefirst bottoms stream to produce purified propylene glycol monomethylether as a second overhead stream, wherein the reaction and/or one ormore of the distillation steps occur in the presence of 0.05 to 1000ppm, based upon the total weight of the alkoxylation mixture, of sodiumborohydride, and wherein the sodium borohydride reduces theconcentration of the component having a carbonyl functional group. 15.The process of claim 14 wherein the alkali metal methoxide catalyst ispotassium methoxide.
 16. The process of claim 14 wherein the alkalimetal methoxide catalyst is sodium methoxide.
 17. The process of claim14 wherein the first overhead stream is recycled back to the reactionstep (a).
 18. The process of claim 14 wherein the sodium borohydride isadded into the alkoxylation mixture prior to distillation step (b). 19.The process of claim 14 wherein the sodium borohydride is added into thefirst bottoms stream prior to distillation step (c).
 20. The process ofclaim 14 wherein the purified propylene glycol monomethyl ether has a UVabsorbance, at 245 nm, of 1 or less.